FIG. 1 illustrates a conventional signal monitoring system 100. The signal monitoring system 100 includes a differential resistor-capacitor (RC) low-pass filter (LPF) 102 and a differential analog-to-digital converter (ADC) 104. The differential ADC 104 includes input terminals INP and INN coupled to the differential RC LPF 102. The RC LPF 102 receives an input signal VIN, blocks or attenuates high-frequency noises mixed in the input signal VIN, and passes a low-frequency portion V′IN of the input signal VIN to the ADC 104. The RC LPF 102 can generate a pair of output signals VOUT1 and VOUT2, and control a difference between the output signals VOUT1 and VOUT2 to be equal to the V′IN multiplied by a gain g102 of the RC LPF 102, e.g., VOUT2−VOUT1=VIN*g102. The ADC 104 can receive a differential signal VD, e.g., VD=VOUT2−VOUT1, and generate a digital signal 106 indicative of the differential signal VD.
However, in the conventional signal monitoring system 100, there is a tradeoff between the system response speed and the monitoring accuracy. More specifically, according to the characteristics of a low-pass filter, a dominant pole fp (or a first pole) of the RC LPF 102 is given by: fp=1/(2π*R*C), where R represents an equivalent resistance of the RC LPF 102, and C represents an equivalent capacitance of the RC LPF 102. Since the equivalent resistance and the equivalent capacitance of the RC LPF 102 are constant, the dominant pole fp of the RC LPF 102 is also constant. Thus, the bandwidth of the RC LPF 102 that is determined by the dominant pole fp is also constant.
On one hand, in order to increase the response speed for the RC LPF 102 to vary the output signals VOUT1 and VOUT2 according to a variation of the input signal VIN, the bandwidth of the RC LPF 102 needs to increase. However, the larger the bandwidth of the RC LPF 102, the more the noise mixed in the input signal VIN can be passed to the ADC 104. In other words, increasing the bandwidth of the RC LPF 102 filter can decrease the monitoring accuracy of the signal monitoring system 100. On the other hand, in order to decrease the noise passed to the ADC 104, the bandwidth of the RC LPF 102 needs to decrease. However, the narrower the bandwidth of the RC LPF 102, the slower the response speed of the RC LPF 102. Thus, there is a tradeoff between response speed and monitoring accuracy. It is difficult for the conventional signal monitoring system 100 to enhance both response speed and monitoring accuracy.
In addition, if the input signal VIN ranges from 0V to 5V, and the gain g102 of the RC LPF 102 is equal to one, then the differential signal VD from the RC LPF 102 to the ADC 104 also ranges from 0V to 5V. Since the ADC 104 is a bipolar input ADC, the differential input range of the ADC 104 is at least −5V to 5V. If the ADC 104 is a 12-bit ADC, the least significant bit (LSB) of the ADC 104 is given by: LSB=5/211=2.44 mV. The larger the LSB, the lower the measurement precision of the ADC 104. However, in the signal monitoring system 100, half of the differential input range of the ADC 104, e.g., the range from −5V to 0V, is wasted.